Study On Sedimentary Facies And Diagenesis Of Chang-2 Oil-bearing Beds Of The Upper Triassic Yanchang Formation In Yanchang Region

Yanchang Formation of the Upper Triassic in Yanchang region is mainly terrigenous clastic fluvial system deposited under the broad shallow-water platform environment. Chang-2 oil-bearing beds of the Yanchang Formation, which is one of the most important oil-producing beds in the area, is mainly composed of braided river deposits with the meandering river existed only in the restricted northwest part of Yanchang. The channel microfacies broadly distributed in the area with the trend of NE-SW direction, which are alternated with the flood plain microfacies. The sedimentary patterns of the three sub-beds of Chang-2 oil-bearing beds are basically the same.High heterogeneity of the reservoir quality of the Chang-2 oil-bearing beds reflected primarily in the facies, subfacies and microfacies, and the fluid unit of the microfacies as well. Differences among the subfacies or microfacies displayed in terms of petrology, texture, structure and so on. The grain size and dimension of the sandstones in the braided system is larger and brander than those of the meandering system. Furthermore, sorting of the sandstones in the braided river is better compared with that of in the meandering river system. In the same subfacies, grain size is larger and sorting is better in the channel sandstones which developed with large-scale cross-beddings such as trough cross-bedding, compared with sandstones in the flood microfacies. In the same microfacies, better sorting and coarser grained sandstones are distributed in the middle and lower part of the unit.The study indicates that sedimentary facies is one of the most important factors which control the reservoir quality. Grain size of sandstones, a major indication for sedimentary facies, influences the reservoir quality of the sandstones. The coefficient of the grain size between the porosity and permeability is 0.5 and 0.52, respectively. The reservoir quality of the braided deposits (av. 14.8% and 65.7 X10-3 u m2) is better than that of the meandering deposits (av. 11.25% and 25.8X10-3u m2). Similarly, correlation of grain size between porosity and permeability also shows in the microfacies and the fluid unit. The channel sandstones are as efficient reservoirs for its better reservoir quality, while the flood plain as the sealed beds. The sandstones distributed in the upper and lower part in a microfacies usually are with better reservoir property.Chang-2 oil-bearing beds of the Yanchang Formation experienced three diagenetic stages of early diagenesis, late diagenesis and epidiagenesis. The major diagenetic events include mechanical compaction, chemical compaction, cementation, replacement and dissolution. The reservoir quality experienced from being destroyed to being enhanced during the whole diagenetic evolution process. The mechanical compaction is a main factor, which caused the loss of porosity. Chemical compaction and cementation are the other two important factors caused the loss of porosity. The main cements include carbonate and quartz overgrowth. In the cements, carbonate and quartz overgrowth are the main cements, which reduced the reservoir quality of the sandstones. The coefficient of carbonate and quartz overgrowth cements between the porosity and permeability is -0.5773 and -0.3915, -0.5773 and -0.3915, respectively. The acidic liquid related to decomposition of organic mater during hydrocarbon enrichment in the late diagenesis, and meteoric-waters duringepidiagenesis, had resulted in dissolution of detrital feldspars and carbonate cements, which contributed a better retention of reservoir quality for the sandstones.